"This is probably going to be the worst die-off in history," said Martine DeWit, a veterinarian who oversees the state's marine mammal pathology laboratory.

The record for manatees killed by red tide was set in 1996 with 151 killed by a toxin in the algae bloom. As of Friday, the number killed this year hit 149, and the number could surpass the current record by the end of the weekend.

DeWit said the toxins in the bloom likely settled onto the sea grass that manatee eat, causing them to become paralyzed and eventually drown. The grass beds will also likely retain their poisonous coating for another two months.

Nearly a dozen manatees have been rescued and are being treated at Tampa's Lowry Park Zoo.

Red tide turns the water into a rust color, releasing large amounts of toxins. The current bloom affects 70 miles of the southwest Florida coast, from Sarasota through the middle of Lee County.

Estimates show there are between 4,000-5,000 manatees in Florida, and less than half of them are found in southwest Florida, according to the Tampa Bay Times report. Virtually all of the manatees killed by the red tide have turned up in the center of that stretch.